his elevator doesnt go to the top floor

His elevator doesnt go to the top floor is a slightly more polite way of saying someone isnt very smart. This is also another way to say someone may be mentally incapable of rational and logical thoughts, but it most often related to someones lack of intellect. By using an elevator going to the top floor in the idiom, they are talking about top floor as in the brain, which means the intelligence level of the brain remains low.